Like someone else pointed out, you could try fumble. Some forms also have swipe (disarm). You could try using wands/scrolls to drop his strength and thereby disarming him if you don't have a form with a skill that can do so. I'm sure other players might be able to come up with more ideas, too.

Ultimately though, part of playing a shapeshifter is accepting the fact that the RNG might give you forms that just aren't well suited for fighting certain builds, then getting creative about it.

There are dozens of scenarios where you'll have a form, or worse, two forms that are plain outgunned by a build. Sometimes it's as simple as needing dam reduction on a dodgy form and other times it's being stuck with poisonous bite as your primary attack against a guy with Quaggoth blood. The scenarios are endless.

You benefit from that dynamic as well when you go up against a guy whose build is PK fodder against fly/to or pounce or some other aspect of your form.

If you're looking for a class that allows for serious customization (that you actually control), especially on the fly, shapeshifter probably isn't for you.